There are many kinds of powered, implantable medical systems, e.g., stimulating systems for spinal cord stimulation for pain management, stimulators for controlling urinary incontinence, deep brain stimulators for treating brain disorders such as Parkinson's Disease, and cochlear stimulators for restoring hearing to the profoundly deaf. Other implantable systems include drug infusion pumps, cardiac defibrillators and pacemakers. The foregoing merely illustrates some examples of powered, implantable systems and is not intended to provide an exhaustive list.
By way of example, spinal cord stimulation (SCS) is a well accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an implanted pulse generator, lead wires, and electrodes connected to the lead wires. The pulse generator generates electrical pulses that are delivered to the dorsal column fibers within the spinal cord through the electrodes which are implanted along the dura of the spinal cord. In a typical situation, the attached lead wires exit the spinal cord and are tunneled around the torso of the patient to a sub-cutaneous pocket where the pulse generator is implanted.
The dominant type of SCS system that is commercially available incorporates an implantable device having a primary (one-time-use-only) battery that cannot be recharged once the battery is depleted. Such an implantable device tends to be very large in order to accommodate the primary battery. Implanting such a large device usually involves invasive surgical implantation. In addition, a device containing a primary battery must be explanted from the body once the battery reaches its useful end of life. Energy intensive applications, e.g., ones employing a stimulator having many stimulation channels can quickly deplete the battery and require that the stimulator be explanted after only a short implant period.
The second type of system employs an implantable device that does not require an internal power source and, instead, derives power from an externally transmitted RF source that is transferred transcutaneously (non-invasively) through a pair of coils, one which is implanted and another which is external to the body. In U.S. Pat. No. 3,724,467, an electrode implant is disclosed for the neuro-stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided with a plurality of electrodes formed thereon. The electrodes are connected by leads to an RF receiver, which is also implanted, and which is controlled by an external controller. The implanted RF receiver has no power storage means, and must be coupled to the external controller in order for neuro-stimulation to occur.
The third type of system includes an implantable device that has a power source such as a storage capacitor or a rechargeable battery, herein referred to as a “replenishable” power source. This replenishable power source can be charged from time to time using RF transmission, as described above, i.e., non-invasively through the skin using an implanted coil and an external coil.
Such an implantable system having a replenishable power source (usually a rechargeable battery) offers some unique advantages. One advantage is that a rechargeable battery can generally last much longer than a primary battery. Nevertheless, even a rechargeable battery has a finite life, as the battery can only be recharged so many times before reaching its end of useful life. An important design advantage afforded by using a rechargeable power source is that the implantable device can be made much smaller than a device containing a primary battery. While battery design and capacity have improved greatly, despite those advances, the physical bulk of a primary battery can take up more than half the total internal volume of an implantable device. Consequently, for energy intensive applications, the primary battery employed is generally large and therefore any device containing such a battery is necessarily also large.
Decreasing the implantable device size is extremely advantageous in medical applications because a smaller device is much easier to implant, causes less trauma during surgical implantation and, once implanted, is physically less intrusive. Advanced Bionics® Corporation has succeeded in miniaturizing their BION® neurostimulator to a size small enough to be implanted through a large diameter, hypodermic needle. The small stimulator size and unobtrusive implantation method greatly reduces surgical trauma, the susceptibility to infection, and the time needed for post-operative healing. To keep the overall device size small, the BION® neuro-stimulator can be powered by a rechargeable battery which can be charged non-invasively using a pair of coils: an implanted primary coil that is inside or attached onto the device and an external, secondary transmitting/receiving coil.
A disadvantage, however, of an implantable system having a replenishable power source such as a rechargeable battery is that the user must recharge the battery regularly as it becomes depleted of charge. Such a requirement to recharge the battery can be problematic because it requires patient compliance to timely recharge the battery and, in addition, the patient must be alerted when the battery requires recharging.
What is needed therefore is a system and method that can non-invasively obtain various status data concerning the replenishable power source within the implantable medical device and convey such information to a patient or clinician.